1. Field of the Invention
The invention relates to an optical transmission apparatus, an optical transmission method, and an optical transmission program for phase modulating continuous light using an external modulator.
2. Description of the Related Art
An optical fiber communication has a first request for increasing the speed of a bit rate per one wavelength to carry out a large capacity communication. However, since an optical fiber has dispersion characteristics, a transmission distance is restricted in proportion to the square of a symbol rate. Thus, Patent Literature 1, for example, introduces a transmission method of reducing the symbol rate by a polarization separation method using DP-DPSK (Dual-Polarization Differential Phase-Shift Keying). The reduction of the symbol rate contributes also to the constriction of an occupied band and further contributes to an increase of the overall transmission capacity in a wavelength division multiplex transmission.
Note that DPSK of Patent Literature 1 means differential binary phase-shift keying (differential BPSK: Differential Binary Phase-Shift Keying). The BPSK is a method of modulating the phase of light to bit “0” and “π” according to the bit of a modulation signal “0” and “1”, and DPSK is a method of modulating the phase of light by the difference between the bit of a modulation signal and the bit of a modulation signal just before the above modulation signal. For example, when a bit changes, the phase of light is set to +π, whereas when a bit does not change (the same codes continue), the phase of light is set to −π.
To prevent interference at the time of reception, DP-DPSK modulation requires a first method of employing polarization tracking, a second method of generating signal light using plural light sources, and a third method of giving offset to the carrier frequencies of two orthogonal polarization waves. However, the first method has a problem that it is difficult to perfectly remove interference. The second method has a problem that it is difficult to manage the plural light sources. The third method has a problem that it is weak to the intercode interference due to band restriction and the like.
In Patent Literature 1, to solve the problem of the DP-DPSK modulation, the light from the same light source is polarization-separated, one of the lights is DPSK modulated, and the other light is zero chirp π/2 shift DPSK modulated, thereby the lights are polarization multiplexed. The zero chirp π/2 shift DPSK modulation of Patent Literature 1 is DPSK modulation in which a signal point on In-phase channel (I-ch) and a signal point on Quadrature phase channel (Q-ch) are alternately set every one symbol.
In contrast, the optical fiber communication has plural modulation methods, and each of them has an advantage.
FIG. 1 is a block diagram explaining a transmitter 51 for carrying out the DPSK modulation introduced in Patent Literature 2. The transmitter 51 is configured to include a precoder 201, a driver 211, and a LN (LN: Lithium Niobate) modulator 401.
The precoder 201 receives an electric signal to be transmitted, encodes the signal so that it satisfies the standard of an optical signal to be output, and generates a modulation signal. The driver 211 amplifies the modulation signal that is output by the precoder 201 to a voltage necessary to modulation. The LN modulator 401 outputs modulated light by shifting the phase of the optical carrier wave that is output from a laser diode 215 to 0 or π according to the modulation signal output from the driver 211. A constellation map 225 shown in FIG. 1 shows the phase of an ordinary DPSK signal (a transmission signal output from a DPSK optical transmission apparatus). As shown in the constellation map 225, the phase of the DPSK signal is set to 0 or π every symbol. Note that the same operation is carried out also in BPSK modulation (Binary Phase Shift Keying), the difference is only a modulation signal.
FIG. 2 is a block diagram explaining a transmitter 52 for carrying out DQPSK modulation (Differential Quadrature Phase-Shift Keying) introduced in Patent Literature 2. The transmitter 52 is configured to include a precoder 204, drivers (421, 422), and a LN modulator 410.
The LN modulator 410 includes a first arm 431, a second arm 432, a first Mach-Zehnder type modulator 411, a second Mach-Zehnder type modulator 412, and a phase shift section 413. The first Mach-Zehnder type modulator 411 is provided to the first arm 431, and the second Mach-Zehnder type modulator 412 is provided to the second arm 432. The phase shift section 413 is provided behind the Mach-Zehnder type modulator 412 of the second arm 432.
The precoder 204 receives an electric signal to be transmitted and encodes the signal so that it satisfies the standard of an optical signal to be output, and generates an I phase side modulation signal and a Q phase side modulation signal. The drivers (421, 422) amplify the I phase side modulation signal and the Q phase side modulation signal output by the precoder 204 up to a voltage necessary to modulation, respectively and output the signals to the first Mach-Zehnder type modulator 411 and the second Mach-Zehnder type modulator 412 as modulation signals.
The LN modulator 410 branches the optical carrier wave output from the laser diode 215 and outputs the branched optical carrier waves to the first arm 431 and the second arm 432.
The first Mach-Zehnder type modulator 411 provided with the first arm 431 and the second Mach-Zehnder type modulator 412 provided with the second arm 432 output modulation light by shifting the phase of the optical carrier wave to 0 or π according to the I phase side modulation signal and the Q phase side modulation signal, respectively.
Further, in the second arm 432, the phase shift amount of the optical signal by the phase shift section 413 is set to π/2 according to the value of a bias α. Thus, after the phase of the optical signal output from the second Mach-Zehnder type modulator 412 has been shifted π/2, the optical signal is synthesized (multiplexed) with the optical signal output from the first arm 431.
In FIG. 2, 415 shows the constellation of the optical signal output from the first arm 431, and 416 of FIG. 2 shows the constellation of the optical signal output from the second arm 432. Further, 226 of FIG. 2 shows the constellation of the optical signal with a quadrapture phase that is obtained by synthesizing the optical signal output from the first arm 431 with the optical signal output from the second arm 432. Note that the same operation is carried out also in QPSK modulation (Quadrature Phase-Shift Keying), the difference is only a modulation signal.
The optical fiber communication has also a second request that plural modulation methods can be flexibly switched by a single system in consideration of versatility. However, constructing an optical fiber communication system to each of the modulation methods to allow plural modulation signals to be transmitted inevitably increases cost.
Thus, Patent Literature 2 explains a system capable of flexibly switching two modulation methods of DPSK and DQPSK by a single optical fiber communication system. In Patent Literature 2, a single QPSK modulator is used and the two modulation methods of DPSK and DQPSK are switched by bias controlling the modulator.